The Heart

• The right side of the heart pumps deoxygenated blood to the lungs and the left side pumps deoxygenated blood to the whole body
• The left ventricle of the heart has thicker, more muscular walls than the right ventricle - this allows it to contract more powerfully and pump blood all the way around the body

  This right side is less muscular so its contractions are only powerful enough to pump blood to the nearby lungs
• The ventricles have thicker walls than the atria therefore they can push blood out of the heart, whereas the atria just need to push blood a short distance into the ventricles
• The atriventricular valves link the atria to the ventricles and stop blood flowing back into the atria when the ventricles contract
• The semi-lunar valves link the ventricles to the pulmonary artery and aorta, and stop blood flowing back into the heart after the ventricles contract
• The cords attach the atrioventricular valves to the ventricles to stop them being forced up into the atria when the ventricles contract
• The valves only open one way - whether they're open or closed depends on the relative pressure of the heart chambers

  If there's higher pressure behind a valve its forced open, but if pressure is higher in front of the valve its forced shut

  This means that the flow of blood is unidirectional - inly flows in once direction
• The cardiac cycle is an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps the blood continuously circulating round the body

  The volume of the atria and ventricles change as they contract and relax

  Pressure changes also occur due to the changes in chamber volume
• Ventricles relax, atria contract

  • The ventricles are relaxed
  • The atria contract decreasing the volume of the chambers increasing the pressure inside the chambers
  • This pushes the blood into the ventricles
  • There's a slight increase in ventricular pressure and chamber volume as the ventricles receive the ejected blood from the contracting atria
• Ventricles contract, atria relax

  • The atria relax
  • The ventricles contract increasing their pressure
  • The pressure becomes higher in the ventricles than the atria, which forces the atrioventricular valves shut to prevent back-flow
  • The pressure in the ventricles is also higher than in the aorta and pulmonary artery which forces open the semi-lunar valves and blood is forced out into these arteries
• Ventricles relax, atria relax

  • The ventricles and the atria both relax
  • The higher pressure in the pulmonary artery and aorta closes the semi-lunar valves to prevent back flow into the ventricles
  • Blood returns to the heart and the atria fill again due to the higher pressure in the vena cava and pulmonary vein
  • In this turn this starts to increase the pressure of the atria
  • As the ventricles continue to relax, their pressure falls below the pressure of the atria and so the AV valves open
  • This allows blood to flow passively into the ventricles from the atria
  • The atria contract and the whole process begins again